When do we use online as one word and when as two words?
For example, do we say :"I want to go online or on line?"

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    For your sense (connected to a network / the Internet, I assume), it's always the single-word form. You might receive a phone call on line [number] one, but that's not a very common usage. Jul 23 '16 at 19:36

When the internet was more of a novelty, it seems like both forms were used. The two words were often hyphenated as well. For example, the following is a screen shot from a 1997 book entitled The Future of Money in the Information Age.


And a 1992 book reads:

Once you have a valid BBS number, it's time to dial it and get on line.

That said, it looks like the single-word form is winning out, though. It's far easier to find examples where online is a single word. Check out the Ngram.

It may be worth noting that such neologisms are not uncommon in the technology realm; here is a similar Ngram showing how username is becoming a single word.


Online = noun (place)

He searched online. - As in internet.

on = preposition, line = noun (thing)

Generally, articles are placed in front of nouns (a, an, the)

The clothes are on THE line. ["The line" refers to a clothes line]

She had her pens in a line. ["in" = preposition, "line" = direction]

Ms. Johnson had our class line up. [line used as a verb]

Articles: a - placed before a nonspecific noun "Give me a pen.", "Where is a bathroom?" <--- Not a specific pen/bathroom

an - same as above, but if the noun begins with a vowel use "an" "I would like an ice cream cone.", "What is an apple?"

the - placed before a specific noun "The pen is broke.", "Where are the keys?" <---- Specific pen/keys

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